Because of the way non-tipped, jacketed bullets are formed, measuring overall. Cartridge length is almost a waste of time. Even the best bullets vary too much in length. So measuring cartridge base to ogive provides a much more accurate point of reference to quality check loaded rounds.

When used as a way to adjust seating depth, comparing to +/- contact with the lands, it’s no different than adjusting bullet distance in front of the case for breech seating.

Most quality cast bullets vary less in length and from base to ogive than jacketed bullets, But if you are looking for a consistent seated depth, and or jump the process has merit.

There was a thread a couple weeks and I posted targets of low grade 22 LR groups that were straight out of the box and also sorted base to ogive. The sorted ammo had about 25% less vertical.

With compressed loads with fixed cartridges it seams to be a bit more accurate than overall length, but it all depends on the uniformity of the bullets to begin with.

Gentlemen, Thank you for your renewed interest in my original post.

I'm afraid in my ignorance I miss labelled the insert calling it the Anvil (of a comparator which is indeed flat), instead of an insert. In addition perhaps my question was buried in too much 'waffle'.

In fact Dellet has answered my question. Many thanks.

I am aware that there are many available inserts (given the difficulty of importing anything firearms related from the US to the UK), but none that I could find relating to the Sharps 45/70 i have just acquired.

The second question was concerning the Caldwell Ballistic Chronograph to which i have access, so not particularly important. 

Thanks again,

John D

boats wrote on May 28^th, 2020 at 9:09am:


The idea of a comparator is not really to set seating depth, only to check it.

You start with any of the methods talked about to find the lands, then measure that cartridge.

Then set up your seating die based on that measurement using the tool, and whether you want say .015” jump or jam. Once you’ve found your accuracy length, it become easier to repeat setting up the press.

When you want to start spilling hairs, then sort your bullets first, base to ogive. Then the loaded length will be a tighter tolerance. Seated depth/compression, as well as jump or engraved depth into the bore.

The other thing that can be done is to save the original bullet and case used to take the measurement. You can use it to measure throat erosion later on.

Dellet hit the nail on the head.

Boats

Back in the 80s when I started loading BPCR, I was told by all the local gurus to load with  bullet pushing tight to the lands.  About 2 tons of BP later I decided to try a little jump just to see.  My groups cut in half!    Leave no stone upturned